Paypal BuyNow - GET instead of POST with params for returnUrl

I´m running into an issue that I detected today. I have the following paypal button config :

<form action="https://www.paypal.com/cgi-bin/webscr" method="post" i d="paypalFormId">
    <input type="hidden" name="cmd" value="_xclick">
    <input type="hidden" name="business" value="mybusinessID">
    <input type="hidden" name="lc" value="DE">
    <input type="hidden" name="button_subtype" value="services">
    <input type="hidden" name="no_note" value="1">
    <input type="hidden" name="no_shipping" value="2">
    <input type="hidden" name="item_name" value="MyProductName">
    <input type="hidden" name="amount" value="1.00">
    <input type="hidden" name="custom" value="somecustom infos needed in the redirection">
    <input type="hidden" name="rm" value="2">
    <input type="hidden" name="return" value="https://myreturnurl.com">
    <input type="hidden" name="cancel_return" value="https://mycancelurl.com">
    <input type="hidden" name="currency_code" value="EUR">
    <input type="hidden" name="notify_url" value="myIPNUrl">
    <input type="hidden" name="bn" value="PP-BuyNowBF:btn_buynowCC_LG.gif:NonHosted">
    <div class="row">
       <button type="submit" class="btn btn-submit-next pull-right">Bestellung abschließen</button>
    </div>

I didn´t change anything that could impact this code in the last days, nevertheless, redirection POST parameters are missing and redirection is a GET now and not a POST method anymore, even with the rm=2 !

Any idea, what´s going on ?

Best regards,

Antoine

Edited :

I was able to isolate the issue on JSFiddle : if I remove the item_name input then it works. I have no clue why but maybe it helps you guy telling me what I do wrong !